nix: use store paths for propagated GStreamer packages

[romen]

What do you want to change?

I'd like to fix an issue that appears when trying to run nix shell github:ghostty-org/ghostty/v1.4.0 (and also on current main).

You can preview the (trivial) fix at: romen@539a7f1

The commit message also has a succint explanation of the problem.

Why do you want to make this change?

This bug affects users that want to use nix shell (rather than nix run) to access a specific version of ghostty.
nix run use is not affected, since propagated-user-env-packages is not evaluated in that case.

I acknowledge that:

• I have read the Contributing Guide and understand the contribution process.
• I have read and agree to follow the AI Usage Policy.
• I wrote this vouch request myself, in my own voice, without AI generating it.

[romen]

The (closed) draft PR at #14929 includes a full description of the issue, steps taken to fix it and to test it, and the AI disclosure.
I opened that draft PR fully knowing it would be automatically closed, as part of the vouching mechanism, but I preferred to store the description and details there while the issue/fix was fresh rather than forgetting most of it by the time the vouching process goes through.
I assumed that given the bot would close the unvouched PR, doing that won't burden human reviewers.

[pluiedev]

!vouch